Large array of upward pointing p-i-n diodes having large and uniform current

ABSTRACT

An upward-pointing p-i-n diode formed of deposited silicon, germanium, or silicon-germanium is disclosed. The diode has a bottom heavily doped p-type region, a middle intrinsic or lightly doped region, and a top heavily doped n-type region. The top heavily doped p-type region is doped with arsenic, and the semiconductor material of the diode is crystallized in contact with an appropriate silicide, germanide, or silicide-germanide. A large array of such upward-pointing diodes can be formed with excellent uniformity of current across the array when a voltage above the turn-on voltage of the diodes is applied. This diode is advantageously used in a monolithic three dimensional memory array.

RELATED APPLICATIONS

This application is related to Herner, U.S. patent application Ser. No.11/692,151,“Method to Form Upward-Pointing P-I-N Diodes Having Large andUniform Current”; to Herner et al, U.S. patent application Ser. No.11/692,144, “Method to Form a Memory Cell Comprising a Carbon NanotubeFabric Element and a Steering Element”; and to Herner et al, U.S. patentapplication Ser. No. 11/692,148, “Memory Cell Comprising a CarbonNanotube Fabric Element and a Steering Element,” all filed on even dateherewith and hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

A diode has the characteristic of allowing very little current flowbelow a certain turn-on voltage, and substantially more current abovethe turn-on voltage. It has proven difficult to form a large populationof vertically oriented p-i-n diodes having a bottom heavily doped p-typeregion, a middle intrinsic region, and a top heavily doped n-type regionwith good uniformity of current among the diodes when a voltage abovethe turn-on voltage is applied.

It would be advantageous to form a large population of suchupward-pointing diodes having good uniformity, specifically for use in amemory array.

SUMMARY OF THE PREFERRED EMBODIMENTS

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Ingeneral, the invention is directed to a population of upward-pointingp-i-n diodes.

A first aspect of the invention provides for a first device level formedabove a substrate comprising a plurality of vertically oriented p-i-ndiodes, each p-i-n diode comprising a bottom heavily doped p-typeregion, a middle intrinsic or lightly doped region, and a top heavilydoped n-type region, wherein each p-i-n diode has the form of a pillar,wherein, for at least 99 percent of the p-i-n diodes, current flowingthrough the p-i-n diodes when a voltage between about 1.5 volts andabout 3.0 volts is applied between the bottom heavily doped p-typeregion and the top heavily doped n-type region is at least 1.5microamps, wherein the p-i-n diodes comprise deposited silicon,germanium, or silicon-germanium, wherein the first plurality of p-i-ndiodes includes every p-i-n diode in the first device level.

Another aspect of the invention provides for a first memory levelcomprising a first plurality of memory cells, each first memory cellcomprising a vertically oriented p-i-n diode in the form of a pillar,each vertically oriented p-i-n diode comprising a bottom heavily dopedp-type region, a middle intrinsic or lightly doped region, and a topheavily doped n-type region, wherein the first memory cells compriseprogrammed cells and unprogrammed cells, wherein at least half of thememory cells are programmed cells, wherein current flowing through thep-i-n diodes of at least 99 percent of the programmed cells when avoltage between about 1.5 volts and about 3.0 volts is applied betweenthe bottom heavily doped p-type region and the top heavily doped n-typeregion is at least 1.5 microamps, wherein the first plurality of memorycells includes every memory cell in the first memory level.

Each of the aspects and embodiments of the invention described hereincan be used alone or in combination with one another.

The preferred aspects and embodiments will now be described withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an embodiment of a memory cell describedin the '030 patent.

FIG. 2 is a perspective view of a portion of a first memory levelcomprising memory cells like the cell of FIG. 1.

FIG. 3 a is a perspective view showing two stacked memory levels sharingconductors. FIG. 3 b is a cross-sectional view of the same structure.FIG. 3 c is a cross-sectional view showing two stacked memory levels notsharing conductors.

FIG. 4 a is a probability plot of current at applied voltage of 2 voltsfor a population of downward-pointing diodes formed according to anembodiment of the '030 patent. FIG. 4 b is a probability plot of currentat applied voltage of 2 volts for a population of upward-pointing diodesformed according to an embodiment of the '030 patent.

FIG. 5 is perspective view of an embodiment of the present invention.

FIG. 6 is a probability plot of current at applied voltage of 2 voltsfor a population of upward-pointing diodes formed according to thepresent invention.

FIGS. 7 a-7 d are cross-sectional views illustrating stages in formationof two memory levels, the first memory level including upward-pointingdiodes formed according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the '470 application, the '030 patent, and the '549 application, allowned by the assignee of the present invention, memory cells aredescribed, each including a vertically oriented p-i-n diode in the formof a pillar. Such a diode is formed of a semiconductor material such assilicon, germanium, or a silicon-germanium alloy, and has a bottomheavily doped region of a first semiconductor type, a middle intrinsicor lightly doped region, and a top heavily doped region of a secondsemiconductor type opposite the first. It has been described to formthis diode in both orientations, either having a bottom heavily dopedp-type region and a top heavily doped n-type region; or the reverse,with a bottom heavily doped n-type region and the top heavily dopedp-type region.

FIG. 1 illustrates a memory cell formed according to an embodiment ofthe '030 patent. Such a memory cell includes a bottom conductor 200 anda top conductor 400, with a vertically oriented p-i-n diode 302 and adielectric rupture antifuse 118 arranged electrically in series betweenthem. In its initial, unprogrammed state, when a read voltage, forexample of 2 volts, is applied between bottom conductor 200 and topconductor 400, very little current flows between them. Application of arelatively large programming voltage alters the memory cell, and, afterprogramming, significantly more current flows between bottom conductor200 and top conductor 400 at the same read voltage. This difference incurrent between the unprogrammed and programmed states is measurable,and each can correspond to a distinct data state; for example anunprogrammed cell can be considered to be a data “0” while a programmedcell is a data “1”.

FIG. 2 shows a portion of a first memory level comprising a plurality ofbottom conductors 200, a plurality of pillars 300, each pillar includinga diode and a dielectric rupture antifuse as in FIG. 1, and a pluralityof top conductors 400. Each pillar 300 is disposed between one of thebottom conductors 200 and one of the top conductors 400. Such a memorylevel can be formed above a substrate such as a conventionalmonocrystalline silicon wafer. Multiple memory levels can be formedstacked above the first to form a dense monolithic three dimensionalmemory array.

A diode is a rectifying device, conducting current more readily in onedirection than in the other. A diode can be said to point in itsdirection of preferred conduction. A vertically oriented diode havingn-type semiconductor material at the bottom and p-type semiconductormaterial at the top can be said to be downward-pointing, while avertically oriented diode having p-type semiconductor material at thebottom and n-type semiconductor material at the top can be said to beupward-pointing. Note that in this application, when terms indicatingspatial relationships, like “upward”, “downward”, “above”, “below”, andthe like are used, these terms are relative to the substrate, which isassumed to be at the bottom of the frame of reference. For example, if afirst element is described to be above a second element, the firstelement is farther from the substrate than the second.

In a vertically stacked memory array, it is preferred for verticallyadjacent memory levels to share conductors, as shown in perspective viewin FIG. 3 a, in which the conductors 400 serve both as the topconductors of the first memory level M0 and as the bottom conductors ofthe second memory level M1. The same structure is shown in across-sectional view in FIG. 3 b. FIG. 3 c show a cross-sectional viewof an array in which conductors are not shared. In FIG. 3 c, each memorylevel has bottom conductors (200, 500), pillars (300, 600), and topconductors (400, 700), with an interlevel dielectric separating memorylevels M0 and M1, with no conductors shared. The architecture of FIGS. 3a and 3 b requires fewer masking steps and reduces fabrication costs toproduce the same density of memory cells as shown in FIG. 3 c. Sharingof conductors, as in FIGS. 3 a and 3 b, is most readily achievedelectrically if diodes on adjacent levels point in opposite directions,for example if the first memory level M0 diodes are upward-pointing,while the second memory level M1 diodes are downward-pointing. A stackedarray of only upward-pointing or only downward-pointing diodes willgenerally be formed with conductors not shared, as in FIG. 3 c.

A large memory array will typically include millions of memory cells,each of which must be sensed. There will inevitably be some variation incharacteristics between memory cells in such a large array. To improvereliability, for a large array of memory cells, it is advantageous tomaximize the difference between the unprogrammed and the programmedstates, making them easier to distinguish. It is further advantageous tominimize variation between cells, and for the cells to behave asuniformly as possible.

FIG. 4 a is a probability plot showing unprogrammed current andprogrammed current under the same applied read voltage for a populationof memory cells like those of the '030 patent (shown in FIG. 1)including a diode and an antifuse in series between conductors in whichthe diodes are all downward-pointing; i.e. the diodes have a bottomheavily doped n-type region, a middle intrinsic region, and a topheavily doped p-type region. It will be seen that the unprogrammedcurrent for the downward-pointing diodes, shown on line A, is tightlygrouped close to 10⁻¹² amps. Similarly, the programmed current, shown online B, with the exception of one outlier, is tightly grouped betweenabout 10⁻⁵ and 10⁻⁴ amps. The distributions of unprogrammed current(line A) and programmed current (line B) are spaced well apart from eachother and both are tightly grouped.

FIG. 4 b is a probability plot showing unprogrammed current andprogrammed current for a population of upward-pointing diodes formed asin the '030 patent. The unprogrammed current, shown on line C, is verysimilar to the unprogrammed current of the downward pointing diode, lineA of FIG. 4 a. The programmed current, however, shown on line D, shows amuch wider distribution than the programmed current on line B of FIG. 4a. Programmed current for this upward-pointing diode ranges from about8×10⁻⁸ amps to 7×10⁻⁵ amps, a difference approaching three orders ofmagnitude. A large number of the population of these diodes haveprogrammed current less than 1 microamp. This nonuniformity and lowprogrammed current make the upward-pointing diode of the '030 patent aless advantageous diode for use in a large array than thedownward-pointing diode.

In the present invention, a fabrication technique has been found thatyields a large population of upward-pointing vertically oriented p-i-ndiodes having good uniformity and large programmed current. FIG. 5 showsan example of a memory cell including an upward-pointing diode formedaccording to an embodiment of the present invention. In this memorycell, the diode is paired with a dielectric rupture antifuse, but, aswill be described, the pictured memory cell is only one of many possibleuses for such a diode, and is provided for clarity.

The memory cell includes first conductor 200 and second conductor 400.Disposed between them are dielectric rupture antifuse 118 (shownsandwiched between conductive barrier layers 110 and 111) and diode 302.Diode 302 includes bottom heavily doped p-type region 112, middleintrinsic region 114, and top heavily doped n-type region 116. Diode 302is formed of semiconductor material, for example silicon, germanium, ora silicon-germanium alloy. For simplicity, this semiconductor materialwill be described as silicon. The silicon is preferably predominantlyamorphous as deposited (though p-type region 112, if doped in situ, willlikely be polycrystalline as deposited.) Top heavily doped p-type region116 is doped with arsenic. In preferred embodiments, region 116 isformed by forming middle intrinsic region 114, then doping the top ofmiddle intrinsic region 114 with arsenic by ion implantation. As will beseen, this ion implantation step may take place either before or afterthe patterning and etching step that forms the pillar. In alternativeembodiments, region 116 may be doped in situ by flowing an appropriatesource gas such as AsH₃ during silicon deposition at flows sufficient toresult in an arsenic concentration of at least 5×10²⁰ atoms/cm³. Thebottom layer of top conductor 400 is a silicide-forming metal such astitanium, cobalt, chromium, tantalum, platinum, niobium, or palladium.Titanium and cobalt are preferred; titanium is most preferred. During ananneal performed to crystallize the silicon, the silicide-forming metalreacts with the silicon of top heavily doped n-type region 116 and formsa silicide layer, for example titanium silicide. FIG. 6 is a probabilityplot showing current at a read voltage of about 2 volts for a populationof such upward-pointing diodes; as will be seen, this population hasgood uniformity, with very little variation between diodes, and relativelarge forward current, with median current of about 35.5 microamps. Inparticular, note that programmed current at 2 volts for all diodes inthis population is above about 3 microamps.

As described, memory cells in the array described are sensed by applyinga read voltage across the memory cell. Ideally the read voltage appliedis the same for every memory cell in the array; in practice there willbe some variation due to the location of each memory cell within thearray. For example, cells located farther from sensing circuitry have alonger interconnect than cells located closer to it. The increasedlength of the interconnect results in increased resistance, resulting insmaller voltages across the diodes of more distant cells as compared tocloser ones. Small variations in the read current of the diode due tovariations in the interconnect length, and resistance, are not inherentproperties of the diode of the present invention, however. The termdevice level will refer to a plurality of substantially coplanar devicesformed at the same level above a substrate, and generally by the sameprocessing steps; an example of a device level is a memory levelincluding a plurality of substantially coplanar memory cells formedabove a substrate. In one example, in a device level including apopulation of upward-pointing p-i-n diodes formed according to thepresent invention, the voltage applied across the diode, i.e. betweenthe bottom p-type region and the top n-type region of the diode, isbetween about 1.8 volts and about 2.2 volts for any diode in the devicelevel, regardless of its location; and current flowing through 99percent of the diodes in this device level under this applied voltage isat least 1.5 microamps. In other examples, in the present invention acurrent of about 1.5 microamps is achievable for 99 percent of diodes ina device level when the voltage applied across the diode (between thebottom p-type region and the top n-type region of the diode) is betweenabout 1.1 volts and about 3.0 volts, preferably between about 1.5 voltsand about 3.0 volts, most preferably between about 1.8 volts and about2.2 volts, for example when the semiconductor material is asilicon-germanium alloy such as Si_(0.8)Ge_(0.2). This population ofp-i-n diodes may be a device level having 100,000 p-i-n diodes or more,for example 1,000,000 p-i-n diodes or more.

In preferred embodiments, the device level is a memory level comprisingmemory cells of the present invention, wherein the first memory cellscomprise programmed cells and unprogrammed cells. In such a memoryarray, during use, some cells will be programmed while others areunprogrammed. In a preferred embodiment, when at least half of thememory cells are programmed cells, current flowing through the p-i-ndiodes of at least 99 percent of the programmed cells when a voltagebetween about 1.5 volts and about 3.0 volts is applied between thebottom heavily doped p-type region and the top heavily doped n-typeregion is at least 1.5 microamps, wherein the first plurality of memorycells includes every memory cell in the first memory level. In morepreferred embodiments, the applied voltage is between about 1.8 voltsand about 2.2 volts. This memory level of memory cells may include100,000 cells or more, for example 1,000,000 cells or more, each cellincluding an upward-pointing p-i-n diode formed according to the presentinvention.

The upward-pointing diode of the present invention can advantageously beused in an array of stacked memory levels sharing conductors, mostpreferably having upward-pointing diodes alternating withdownward-pointing diodes on each memory level.

As described in Herner et al., U.S. patent application Ser. No.11/148,530, “Nonvolatile Memory Cell Operating by Increasing Order inPolycrystalline Semiconductor Material,” filed Jun. 8, 2005, herebyincorporated by reference, when deposited amorphous silicon iscrystallized in contact solely with materials with which it has a highlattice mismatch, such as silicon dioxide and titanium nitride, thepolycrystalline silicon or polysilicon forms with a high number ofcrystalline defects, causing it to be high-resistivity. Application of aprogramming pulse through this high-defect polysilicon apparently altersthe polysilicon, causing it to be lower-resistivity.

As described further in the '549 application; as well as in Herner, U.S.Pat. No. 7,176,064, “Memory Cell Comprising a Semiconductor JunctionDiode Crystallized Adjacent to a Silicide”; and in Herner, U.S. patentapplication Ser. No. 11/560,283, “Method for Making a P-I-N DiodeCrystallized Adjacent to a Silicide in Series with A DielectricAntifuse,” filed Nov. 15, 2006, hereinafter the '283 application andhereby incorporated by reference, it has been found that when depositedamorphous silicon is crystallized in contact with a layer of anappropriate silicide, for example titanium silicide, cobalt silicide, ora silicide formed of one of the other named silicide-forming metals, theresulting crystallized silicon is much higher quality, with fewerdefects, and has much lower resistivity. The lattice spacing of titaniumsilicide or cobalt silicide is very close to that of silicon, and it isbelieved that when amorphous silicon is crystallized in contact with alayer of an appropriate silicide at a favorable orientation, thesilicide provides a template for crystal growth of silicon, minimizingformation of defects. Unlike the high-defect silicon crystallizedadjacent only to materials with which it has a high lattice mismatch,application of a large electrical pulse does not appreciably change theresistivity of this low-defect, low-resistivity silicon crystallized incontact with the silicide layer.

In some memory cells using a vertically oriented p-i-n diode, then, asin the '549 application, the diode is formed of higher-defect,higher-resistivity polysilicon, and the memory cell is programmed bychanging the resistivity state of the polysilicon. For thesehigh-defect-diode cells, the data state of the memory cell is storedpredominantly in the resistivity state of the polysilicon of the diode.In other memory cells, as in the '283 application, the diode is formedof low-defect, low-resistivity silicon, is paired with a companionstate-change element (in this case a dielectric rupture antifuse) andthe memory cell is programmed by changing the characteristics of thestate-change element (by rupturing the antifuse, for example.) The termstate-change element is used to describe an element that can take two ormore stable, mutually distinguishable states, usually resistivitystates, and can either reversibly or irreversibly be switched betweenthem. For these low-defect-diode cells, the data state of the memorycell is stored predominantly in the state-change element, not in thestate of the diode. (Note that this discussion has described the use ofsilicon crystallized adjacent to a silicide. The same effect can beexpected for germanium and silicon-germanium crystallized adjacent to agermanide or silicide-germanide.)

The upward-pointing p-i-n diodes of the present invention arecrystallized in contact with a silicide, and are thus of low-defect,low-resistivity semiconductor material. If the upward-pointing diodes ofthe present invention, then, are used in memory cells, they areadvantageously used when paired with a state-change element, for examplean antifuse or a resistivity-switching element. One example of such aresistivity-switching element is a binary metal oxide, such asNi_(x)O_(y), Nb_(x)O_(y), Ti_(x)O_(y), Hf_(x)O_(y), Al_(x)O_(y),Mg_(x)O_(y), Co_(x)O_(y), Cr_(x)O_(y), V_(x)O_(y), Zn_(x)O_(y),Zr_(x)O_(y), B_(x)N_(y), or Al_(x)N_(y), as described in Herner et al.,U.S. patent application Ser. No. 11/395,995, “Nonvolatile Memory CellComprising a Diode and a Resistance-Switching Material,” filed Mar. 31,2006, and hereby incorporated by reference. Another example of aresistivity-switching element is a carbon nanotube fabric, as describedin Herner et al., filed on even date herewith.

Note that the upward-pointing diodes of the present invention mayadvantageously be used in many devices, and is not limited to use inmemory cells; or, if used in memory cells, is not limited to use incells like those specifically described herein.

A detailed example will be provided describing fabrication of a firstmemory level formed above a substrate, the memory level comprisingmemory cells having an upward-pointing diode and high-K dielectricantifuse arranged in series between a bottom conductor and a topconductor, as well fabrication of a second memory level above itcomprising downward-pointing diodes, the two memory levels sharingconductors. Details from the '283 application, and from the otherincorporated applications, may prove useful in fabrication of thismemory level. To avoid obscuring the invention, not all details fromthese or other incorporated documents will be included, but it will beunderstood that none of their teaching is intended to be excluded. Forcompleteness, many details, including materials, steps, and conditions,will be provided, but it will be understood by those skilled in the artthat many of these details can be changed, augmented or omitted whilethe results fall within the scope of the invention.

EXAMPLE

Turning to FIG. 7 a, formation of the memory begins with a substrate100. This substrate 100 can be any semiconducting substrate known in theart, such as monocrystalline silicon, IV-IV compounds likesilicon-germanium or silicon-germanium-carbon, III-V compounds, II-VIIcompounds, epitaxial layers over such substrates, or any othersemiconducting material. The substrate may include integrated circuitsfabricated therein.

An insulating layer 102 is formed over substrate 100. The insulatinglayer 102 can be silicon oxide, silicon nitride, Si—C—O—H film, or anyother suitable insulating material.

The first conductors 200 are formed over the substrate 100 and insulator102. An adhesion layer 104 may be included between the insulating layer102 and the conducting layer 106 to help conducting layer 106 adhere toinsulating layer 102. If the overlying conducting layer 106 is tungsten,titanium nitride is preferred as adhesion layer 104. Conducting layer106 can comprise any conducting material known in the art, such astungsten, or other materials, including tantalum, titanium, or alloysthereof.

Once all the layers that will form the conductor rails have beendeposited, the layers will be patterned and etched using any suitablemasking and etching process to form substantially parallel,substantially coplanar conductors 200, shown in FIG. 7 a incross-section. Conductors 200 extend out of the page. In one embodiment,photoresist is deposited, patterned by photolithography and the layersetched, and then the photoresist removed using standard processtechniques.

Next a dielectric material 108 is deposited over and between conductorrails 200. Dielectric material 108 can be any known electricallyinsulating material, such as silicon oxide, silicon nitride, or siliconoxynitride. In a preferred embodiment, silicon dioxide deposited by ahigh-density plasma method is used as dielectric material 108.

Finally, excess dielectric material 108 on top of conductor rails 200 isremoved, exposing the tops of conductor rails 200 separated bydielectric material 108, and leaving a substantially planar surface. Theresulting structure is shown in FIG. 7 a. This removal of dielectricoverfill to form the planar surface can be performed by any processknown in the art, such as chemical mechanical planarization (CMP) oretchback. In an alternative embodiment, conductors 200 could be formedby a Damascene method instead.

Turning to FIG. 7 b, next optional conductive layer 110 is deposited.Layer 110 is a conductive material, for example titanium nitride,tantalum nitride, or tungsten. This layer may be any appropriatethickness, for example about 50 to about 200 angstroms, preferably about100 angstroms. In some embodiments barrier layer 110 may be omitted.

Next, in this example, a thin layer 118 of a dielectric material ordielectric stack is deposited to form a dielectric rupture antifuse. Inone embodiment, a high-K dielectric, such as HfO₂, Al₂O₃, ZrO₂, TiO₂,La₂O₃, Ta₂O₅, RuO₂, ZrSiO_(x), AlSiO_(x), HfSiO_(x), HfAlO_(x), HfSiON,ZrSiAlO_(x), HfSiAlO_(x), HfSiAlON, or ZrSiAlON, is deposited, forexample by atomic layer deposition. HfO₂ and Al₂O₃ are preferred. IfHfO₂ is used, layer 118 preferably has a thickness between about 5 andabout 100 angstroms, preferably about 40 angstroms. If Al₂O₃ is used,layer 118 preferably has a thickness between about 5 and about 80angstroms, preferably about 30 angstroms. In alternative embodiments,the dielectric rupture antifuse may comprise silicon dioxide.

Conductive layer 111 is deposited on layer 118. It can be anyappropriate conductive material, for example titanium nitride, with anyappropriate thickness, for example about 50 to about 200 angstroms,preferably about 100 angstroms. In some embodiments conductive layer 111may be omitted.

Next semiconductor material that will be patterned into pillars isdeposited. The semiconductor material can be silicon, germanium, asilicon-germanium alloy, or other suitable semiconductors, orsemiconductor alloys. For simplicity, this description will refer to thesemiconductor material as silicon, but it will be understood that theskilled practitioner may select any of these other suitable materialsinstead.

Bottom heavily doped region 112 can be formed by any deposition anddoping method known in the art. The silicon can be deposited and thendoped, but is preferably doped in situ by flowing a donor gas providinga p-type dopant atoms, for example boron, during deposition of thesilicon. In preferred embodiments, the donor gas is BCl₃, and p-typeregion 112 is preferably doped to a concentration of about 1×10²¹atoms/cm³. Heavily doped region 112 is preferably between about 100 andabout 800 angstroms thick, most preferably about 200 angstroms thick.

Intrinsic or lightly doped region 114 can be formed next by any methodknown in the art. Region 114 is preferably silicon and has a thicknessbetween about 1200 and about 4000 angstroms, preferably about 3000angstroms. In general p-type dopants such as boron tend to promotecrystallization; thus the silicon of heavily doped region 112 is like tobe polycrystalline as deposited. Intrinsic region 114, however, ispreferably amorphous as deposited.

Semiconductor regions 114 and 112 just deposited, along with underlyingconductive layer 111, dielectric rupture antifuse 118, and conductivelayer 110, will be patterned and etched to form pillars 300. Pillars 300should have about the same pitch and about the same width as conductors200 below, such that each pillar 300 is formed on top of a conductor200. Some misalignment can be tolerated.

Pillars 300 can be formed using any suitable masking and etchingprocess. For example, photoresist can be deposited, patterned usingstandard photolithography techniques, and etched, then the photoresistremoved. Alternatively, a hard mask of some other material, for examplesilicon dioxide, can be formed on top of the semiconductor layer stack,with bottom antireflective coating (BARC) on top, then patterned andetched. Similarly, dielectric antireflective coating (DARC) can be usedas a hard mask.

The photolithography techniques described in Chen, U.S. application Ser.No. 10/728,436, “Photomask Features with Interior Nonprinting WindowUsing Alternating Phase Shifting,” filed Dec. 5, 2003; or Chen, U.S.application Ser. No. 10/815,312, “Photomask Features with ChromelessNonprinting Phase Shifting Window,” filed Apr. 1, 2004, both owned bythe assignee of the present invention and hereby incorporated byreference, can advantageously be used to perform any photolithographystep used in formation of a memory array according to the presentinvention.

The diameter of the pillars 300 can be as desired, for example betweenabout 22 nm and about 130 nm, preferably between about 32 nm and about80 nm, for example about 45 nm. Gaps between pillars 300 are preferablyabout the same as the diameter of the pillars. Note that when a verysmall feature is patterned as a pillar, the photolithography processtends to round corners, such that the cross-section of the pillar tendsto be circular, regardless of the actual shape of the correspondingfeature in the photomask.

Dielectric material 108 is deposited over and between the semiconductorpillars 300, filling the gaps between them. Dielectric material 108 canbe any known electrically insulating material, such as silicon oxide,silicon nitride, or silicon oxynitride. In a preferred embodiment,silicon dioxide is used as the insulating material.

Next the dielectric material on top of pillars 300 is removed, exposingthe tops of pillars 300 separated by dielectric material 108, andleaving a substantially planar surface. This removal of dielectricoverfill can be performed by any process known in the art, such as CMPor etchback. After CMP or etchback, ion implantation is performed,forming heavily doped n-type top regions 116. The n-type dopant ispreferably a shallow implant of arsenic, with implant energy of, forexample, 10 keV, and dose of about 3×10¹⁵/cm². This implant stepcompletes formation of diodes 302. Note that some thickness, for exampleabout 300 to about 800 angstroms of silicon is lost during CMP; thus thefinished height of diode 302 may be between about 800 and about 4000angstroms, for example about 2500 angstroms for a diode having a featuresize of about 45 nm.

Turning to FIG. 7 c, next a layer 120 of a silicide-forming metal, forexample titanium, cobalt, chromium, tantalum, platinum, niobium, orpalladium, is deposited. Layer 120 is preferably titanium or cobalt; iflayer 120 is titanium, its thickness is preferably between about 10 andabout 100 angstroms, most preferably about 20 angstroms. Layer 120 isfollowed by titanium nitride layer 404. Layer 404 is preferably betweenabout 20 and about 100 angstroms, most preferably about 80 angstroms.Next a layer 406 of a conductive material, for example tungsten, isdeposited; for example this layer may be about 1500 angstroms oftungsten formed by CVD. Layers 406, 404, and 120 are patterned andetched into rail-shaped top conductors 400, which preferably extend in adirection perpendicular to bottom conductors 200. The pitch andorientation of top conductors 400 is such that each conductor 400 isformed on top of and contacting a row of pillars 300, and conductors 400preferably have about the same width as pillars 300. Some misalignmentcan be tolerated.

Next a dielectric material (not shown) is deposited over and betweenconductors 400. The dielectric material can be any known electricallyinsulating material, such as silicon oxide, silicon nitride, or siliconoxynitride. In a preferred embodiment, silicon oxide is used as thisdielectric material.

Referring to FIG. 7 c, note that layer 120 of a silicide-forming metalis in contact with the silicon of top heavily doped region 116. Duringsubsequent elevated temperature steps, the metal of layer 120 will reactwith some portion of the silicon of heavily doped region p-type 116 toform a silicide layer (not shown), which is between the diode and topconductor 400; alternatively this silicide layer can be considered to bepart of top conductor 400. This silicide layer forms at a temperaturelower than the temperature required to crystallize silicon, and thuswill form while intrinsic region 114 and heavily doped p-type region 116are still largely amorphous. If a silicon-germanium alloy is used fortop heavily doped region 116, a silicide-germanide layer may form, forexample of cobalt silicide-germanide or titanium silicide-germanide.Similarly, if germanium is used, a germanide will form.

In the example just described, the diodes 302 of FIG. 7 c areupward-pointing, comprising a bottom heavily doped p-type region, amiddle intrinsic region, and top heavily doped n-type region. Inpreferred embodiments, the next memory level to be monolithically formedabove this one shares conductor 400 with the first memory level justformed; i.e., the top conductor 400 of the first memory level serves asthe bottom conductor of the second memory level. If conductors areshared in this way, then the diodes in the second memory level arepreferably downward-pointing, comprising a bottom heavily doped n-typeregion, a middle intrinsic region, and a top heavily doped p-typeregion.

Turning to FIG. 7 d, next optional conductive layer 210, high-Kdielectric antifuse layer 218, and optional conductive layer 211 areformed, preferably of the same materials, the same thicknesses, andusing the same methods as layers 110, 118, and 111, respectively, ofpillars 300 in the first memory level.

Diodes are formed next. Bottom heavily doped region 212 can be formed byany deposition and doping method known in the art. The silicon can bedeposited and then doped, but is preferably doped in situ by flowing adonor gas providing n-type dopant atoms, for example phosphorus, duringdeposition of the silicon. Heavily doped region 212 is preferablybetween about 100 and about 800 angstroms thick, most preferably about100 to about 200 angstroms thick.

The next semiconductor region to be deposited is preferably undoped. Indeposited silicon, though, n-type dopants such as phosphorus exhibitstrong surfactant behavior, tending to migrate toward the surface as thesilicon is deposited. Deposition of silicon will continue with no dopantgas provided, but phosphorus atoms migrating upward, seeking thesurface, will unintentionally dope this region. As described in Herner,U.S. patent application Ser. No. 11/298,331, “Deposited SemiconductorStructure to Minimize N-Type Dopant Diffusion and Method of Making,”filed Dec. 9, 2005, hereby incorporated by reference, the surfactantbehavior of phosphorus in deposited silicon is inhibited with theaddition of germanium. Preferably a layer of a silicon-germanium alloyincluding at least 10 at % germanium is deposited at this point, forexample about 200 angstroms of Si_(0.8)Ge_(0.2), which is depositedundoped, with no dopant gas providing phosphorus. This thin layer is notshown in FIG. 7 d.

Use of this thin silicon-germanium layer minimizes unwanted diffusion ofn-type dopant into the intrinsic region to be formed, maximizing itsthickness. A thicker intrinsic region minimizes leakage current acrossthe diode when the diode is under reverse bias, reducing power loss.This method allows the thickness of the intrinsic region to be increasedwithout increasing the overall height of the diode. As will be seen, thediodes will be patterned into pillars; increasing the height of thediode increases the aspect ratio of the etch step forming these pillarsand the step to fill gaps between them. Both etch and fill are moredifficult as aspect ratio increases.

Intrinsic region 214 can be formed next by any method known in the art.Region 214 is preferably silicon and preferably has a thickness betweenabout 1100 and about 3300 angstroms, preferably about 1700 angstroms.The silicon of heavily doped region 212 and intrinsic region 214 ispreferably amorphous as deposited.

Semiconductor regions 214 and 212 just deposited, along with underlyingconductive layer 211, high-K dielectric layer 218, and conductive layer210, will be patterned and etched to form pillars 600. Pillars 600should have about the same pitch and about the same width as conductors400 below, such that each pillar 600 is formed on top of a conductor400. Some misalignment can be tolerated. Pillars 600 can be patternedand etched using the same techniques used to form pillars 300 of thefirst memory level.

Dielectric material 108 is deposited over and between the semiconductorpillars 600, filling the gaps between them. As in the first memorylevel, the dielectric material 108 on top of pillars 600 is removed,exposing the tops of pillars 600 separated by dielectric material 108,and leaving a substantially planar surface. After this planarizationstep, ion implantation is performed, forming heavily doped p-type topregions 116. The p-type dopant is preferably a shallow implant of boron,with an implant energy of, for example, 2 keV, and dose of about3×10¹⁵/cm². This implant step completes formation of diodes 602. Somethickness of silicon is lost during the CMP step, so the completeddiodes 602 have a height comparable to that of diodes 302.

Top conductors 700 are formed in the same manner and of the samematerials as conductors 400, which are shared between the first andsecond memory levels. A layer 220 of a silicide-forming metal isdeposited, followed by titanium nitride layer 704 and layer 706 of aconductive material, for example tungsten. Layers 706, 704, and 220 arepatterned and etched into rail-shaped top conductors 700, whichpreferably extend in a direction substantially perpendicular toconductors 400 and substantially parallel to conductors 200.

Preferably after all of the memory levels have been formed, a singlecrystallizing anneal is performed to crystallize the semiconductormaterial of diodes 302, 602, and those diodes formed on additionallevels, for example at 750 degrees C. for about 60 seconds, though eachmemory level can be annealed as it is formed. The resulting diodes willgenerally be polycrystalline. Since the semiconductor material of thesediodes is crystallized in contact with a silicide or silicide-germanidelayer with which it has a good lattice match, the semiconductor materialof diodes 302, 602, etc. will be low-defect and low-resistivity.

In the embodiment just described, conductors were shared between memorylevels; i.e. top conductor 400 of the first memory level serves as thebottom conductor of the second memory level. In other embodiments, aninterlevel dielectric can be formed above the first memory level of FIG.7 c, its surface planarized, and construction of a second memory levelbegun on this planarized interlevel dielectric, with no sharedconductors. In the example given, the diodes of the first memory levelwere upward-pointing, with p-type silicon on the bottom and n-type ontop, while the diodes of the second memory level were reversed, pointingdownward with n-type silicon on the bottom and p-type on top. Inembodiments in which conductors are shared, diode types preferablyalternate, upward on one level and downward on the next. In embodimentsin which conductors are not shared, diodes may be all one type, eitherupward- or downward-pointing. The terms upward and downward refer to thedirection of current flow when the diode is under forward bias.

In some embodiments, it may be preferred for the programming pulse to beapplied with the diode in reverse bias. This may have advantages inreducing or eliminating leakage across the unselected cells in thearray, as described in Kumar et al., U.S. patent application Ser. No.11/496,986, “Method For Using A Memory Cell Comprising SwitchableSemiconductor Memory Element With Trimmable Resistance,” filed Jul. 28,2006, owned by the assignee of the present invention and herebyincorporated by reference.

Fabrication of two memory levels above a substrate has been described.Additional memory levels can be formed in the same manner, forming amonolithic three dimensional memory array.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as a wafer, withno intervening substrates. The layers forming one memory level aredeposited or grown directly over the layers of an existing level orlevels. In contrast, stacked memories have been constructed by formingmemory levels on separate substrates and adhering the memory levels atopeach other, as in Leedy, U.S. Pat. No. 5,915,167, “Three dimensionalstructure memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

A monolithic three dimensional memory array formed above a substratecomprises at least a first memory level formed at a first height abovethe substrate and a second memory level formed at a second heightdifferent from the first height. Three, four, eight, or indeed anynumber of memory levels can be formed above the substrate in such amultilevel array.

An alternative method for forming a stacked memory array in whichconductors are formed using Damascene construction, rather than usingsubtractive techniques as in the examples provided, is described inRadigan et al., U.S. patent application Ser. No. 11/444,936, “ConductiveHard Mask to Protect Patterned Features During Trench Etch,” filed May31, 2006, assigned to the assignee of the present invention and herebyincorporated by reference. The methods of Radigan et al. may be usedinstead to form an array according to the present invention. In themethods of Radigan et al., a conductive hard mask is used to etch thediodes beneath them. In adapting this hard mask to the presentinvention, in preferred embodiments the bottom layer of the hard mask,which is in contact with the silicon of the diode, is preferablytitanium, cobalt, chromium, tantalum, platinum, niobium, or palladium.During anneal, then, a silicide forms, providing the silicidecrystallization template. In this embodiment, the ion implantation stepto form the top heavily doped p-type region takes place before thepatterning step to form the pillars.

In the examples provided so far, the silicide is formed at the topcontact of the diode. In alternative embodiments, it may be formedelsewhere, for example at the bottom contact. For example, the siliconof the diode can be deposited directly on a silicide-forming metal, anda state-change element, such as an antifuse or a resistivity-switchingelement (carbon nanotube fabric or a binary metal oxide, for example)formed on top of the diode.

The upward-pointing diode of the present invention has been described asused in a one-time programmable memory cell (when paired with anantifuse) or in a rewriteable memory cell (when paired with aresistivity-switching element.) It will be understood, however, it isimpractical to list all possible uses of the diode of the presentinvention, and that these examples are not intended to be limiting.

Detailed methods of fabrication have been described herein, but anyother methods that form the same structures can be used while theresults fall within the scope of the invention.

The foregoing detailed description has described only a few of the manyforms that this invention can take. For this reason, this detaileddescription is intended by way of illustration, and not by way oflimitation. It is only the following claims, including all equivalents,which are intended to define the scope of this invention.

1. A first device level formed above a substrate comprising a plurality of vertically oriented p-i-n diodes, each p-i-n diode comprising a bottom heavily doped p-type region, a middle intrinsic or lightly doped region, and a top heavily doped n-type region, wherein each p-i-n diode has the form of a pillar, wherein, for at least 99 percent of the p-i-n diodes, current flowing through the p-i-n diodes when a voltage between about 1.5 volts and about 3.0 volts is applied between the bottom heavily doped p-type region and the top heavily doped n-type region is at least 1.5 microamps, wherein the p-i-n diodes comprise deposited silicon, germanium, or silicon-germanium, wherein the first plurality of p-i-n diodes includes every p-i-n diode in the first device level.
 2. The first device level of claim 1 wherein current flowing through the p-i-n diodes when the voltage applied between the bottom heavily doped p-type region and the top heavily doped n-type region is between about 1.8 volts and about 2.2 volts.
 3. The first device level of claim 1 wherein each diode is in contact with a silicide, germanide, or silicide-germanide layer.
 4. The first device level of claim 3 further comprising: a first plurality of substantially parallel, substantially coplanar rail-shaped conductors formed above the substrate; and a second plurality of substantially parallel, substantially coplanar rail-shaped conductors formed above the first conductors, each first p-i-n diode vertically disposed between one of the first conductors and one of the second conductors.
 5. The first device level of claim 4 wherein the plurality of vertically oriented p-i-n diodes includes at least 100,000 p-i-n diodes.
 6. The first device level of claim 1 wherein a second device level is monolithically formed above the first.
 7. The first device level of claim 1 wherein the substrate comprises monocrystalline silicon. 